In the conventional art of metrological appliances in the relevant field, the type of instrument for measurement of inside diameter is mechanical three-point inside micrometer. This instrument is used currently for direct measurement of inside diameter. The mechanical micrometer head contains three radial legs, which are pushed outward by rotary action of a spiral ramp. The spiral ramp in the micrometer head is rigidly coupled to its operating spindle either directly or through a number of extension rods. The spindle carries a feel ratchet which slips when the radial legs are in contact with inside diameter providing reading of observed diameter.
In the conventional mechanical three-point micrometer as described herein above, such an instrument is very much suitable for local inside diameter measurement, but not suitable for inside diameter measurement at remote locations where straight and direct access is not available for the whole instrument i.e., the micrometer head, extension rods and operating spindle. Moreover, since an operator needs to stand just in front of open end of a pipe/tube being measured by this mechanical micrometer, safety of the operator cannot be ensured in a hazardous working environment. Hence, the application of this instrument is not suitable for service conditions like high temperature area or in under-water application. The existing range of instruments is also not adapted for use in presence of toxic or radioactive materials, which put the operator to health hazards, posing threat of disease/disability/life upon repeated exposures, in sustained operations.
Some other types of instruments in the existing range comprises use of compressed air, ultrasonic signal etc. for indirect measurement of inside diameter. However, these instruments are quite different in operational principle and less reliable and versatile limiting their application to a narrower field. Further, such instruments are not self-centering type, thus not being able to ensure accuracy of measuring mean inside diameter at a particular location. The ultrasonic-type micrometer head contains a number of electronic components and hence, is susceptible to error due to signal drift and noise because of temperature, pressure, radiation etc at the place of measurement.
A fair number of US Patent documents have dealt with the subject of measuring inside diameter of conduits/pipes viz U.S. Pat. No. 6,895,681 titled ‘Method and instrument for measuring inside diameter of conduit’, U.S. Pat. No. 6,675,632 titled ‘Inside diameter measuring method and apparatus, U.S. Pat. No. 5,787,596 titled ‘Apparatus for simultaneously measuring thickness of bottom wall and inside diameter of bottoming hole, U.S. Pat. No. 4,216,586 titled ‘Pipe inside diameter measurement gauge’, U.S. Pat. No. 3,959,887 titled ‘Device for precision measurement of internal diameters’. But none of the cited prior art documents involve any hydraulic system for the apparatus used for measurement of inside diameters, as of the present invention.
Also, to change easily a detection gap length and to improve the detection sensitivity, by comparing and amplifying the back pressure of a detecting nozzle and a reference pressure by a differential pressure amplifier in a hydraulic type micrometer is disclosed in JP 57153207 dated Sep. 21, 1982 titled ‘Hydraulic type micrometer’. It further states that compressed air from an air source has pressure reduced by a reducing valve and is adjusted to a supply pressure. A gauge is used for this adjustment. When air of the supply pressure is jetted onto the surface of a material to be measured on a reference face from a detecting nozzle through a fixed orifice, a backpressure corresponding to a detecting gap is generated in the supply path to the nozzle. The back pressure is supplied to a comparison pressure supply port of a differential pressure amplifier. A reference pressure adjusted by an orifice and a needle valve is supplied to a comparison pressure input port of the differential pressure amplifier, and a pressure switch is operated when the gap length becomes a length corresponding to the reference pressure. Thus this prior art involves the hydraulic operation using compressed air for detecting a gap corresponding to a back pressure when compared against a reference pressure. However, as apparent from the cited disclosure the prior art does not specifically address any simple and user friendly micrometer for ready and safe measurement of inside diameter of pipes/tubes remotely especially for use in measurements involving hazardous location or humanly non-accessible locations.
There has been, therefore, a continuing need in the art for developing micrometer for measuring remotely the inside diameter of pipes/tubes through long straight/bend stretch eliminating the problems associated with inaccessibility due to complexity of measurements by conventional micrometers wherein the spiral ramp in the micrometer head is rigidly coupled to its operating spindle either directly or through a number of extension rods. Moreover, while catering to such need of remote measurement of internal dimensions of objects in hazardous or humanly in accessible locations any such measuring device is further required to ensure reliable and accurate measurement of inside diameter remotely ensuring safety of operator even when applied to operating situation like high temperature or in presence of toxic or radioactive materials. It is also important that such measuring system be simple and user friendly in construction and operation, avoiding use of sensitive/delicate electronic components/devices, so as to eliminate problems/errors as observed in conventional instruments, due to signal drift and noise associated with factors like temperature, pressure, radiation etc at the place of measurement.